The present invention relates to a decoder of an error correcting code, and more particularly to a maximum likelihood decoder such as Viterbi decoder which adds, compares and selects state metric and path metric along each path of a trellis diagram.
The maximum likelihood decoder selects a sequence out of all the transmitted sequences which is most likely to match the received data sequence and determines corresponding digital (data) information. In order to determine the maximum metric, it performs so-called ACS operation (Addition, Comparison and Selection).
In the maximum likelihood decoder, processing time for ACS rapidly increases with the growing number of states of the convolutional code. When the number of states is more than eight, it becomes quite difficult to attain a practical decoder in view of the processing time.